Stainless steel and method



Sept. 30, 1958 TANCZYN 2,854,330

STAINLESS STEEL AND METHOD Filed March 4, 1957 2 Sheets-Sheet 1 3 v Q s 2' 3, %m w Q R u; q 0 I S a; g 9 L k k E E i b s E s (I) v 5 Q E I Q i (L 1? u E Q It Q: S a 4 t g g Q D b Q q Q SUM OF CARBON NITROGEN INVENTOR H iS ATTORNEY Sept. 30, 1958 Filed March 4, 1957 H. TANCZYN STAINLESS STEEL AND METHOD 2 Sheets-Sheet 2 FIG. 2

I I I I I I //VFLUEA/0E 0F TEMPERATURE AND TIME ON THE HARDE/VAB/L/T) 0F GHROM/UM- COPPER 55 STAINLESS STEELS AS COMPARED WITH GHROM/UM- NICKEL-COPPER STAINLESS STEEL.

I 58 HOURS x. 50 N HEAT#/ U) HEA7' HEAT 67- /v/- G'u m QT/ON 900 /000 //00 I200 TREATED HARDE/V/NG TEMPERATURE F INVENTOR Harry Tanczyn HIS ATTORNEY United States Patent STAINLESS STEEL AND METHOD Harry Tanczyn, Baltimore, Md., assignor to Armco Steel Corporation, a corporation of Ohio Application March 4, 1957, Serial No. 643,798

11 Claims. (Cl. 75-125) This invention relates to precipitation-hardenable stainless steel and more particularly to such a steel which is substantially free of nickel. In addition, it relates to a method of hardening the steel and to the resultingprecipitation-hardened steel itself.

One of the objects of the present invention is the provision of a stainless steel which is substantially free of nickel, which possesses good working and forming characteristics in the pre-hardened condition, and which readily may be hardened by precipitation-hardening meth ods to' give high values of yield strength and ultimate strength at room temperatures and-at elevated temperatures with improved stress rupture properties.

Another object is the provision of a precipitation-hardenable steel which is hardenable throughout a substantial range of heat-treating temperatures and throughouta substantial range of treating times withoutgreatly aifecting the hardness achieved,.that is, a steel which is'less sub-w of elements and in the proportioning thereof, as well asin the various operational steps and the relation of each of the same to one or more of theothers as described herein, the scope of the application of which is indicated in the claims at the end of this specification.

In the accompanying drawings:-

Fig. 1 is a diagram setting forth the range of compositionofthe steel of the present:invention in terms of the chrominurn. equivalent versus the carbon equivalent; and

Fig. 2 is a graph illustratinggthe influence of temperature and time on the hardenability of the steels ofmyin vention ascornpared to a steel of the prior art;

As conducive to a better understanding'of certain-aspects of my invention, it is to'be' noted at tlns' point that-the stainless steels generally are vdefinedas containing;v

some 10%-to 35%-chromium with or without nick"e1,f

.andwith or without additions of manganese,silicon,.cop--.

per, cobalt, molybdenum, tungsten, vanadium, titanium, sulphur, selenium and the like for special purposes; with theremainder substantially all iron.v The carbon content ordinarily is on the order of.-0.03.% to 0.20%, although it may range on up to 1% or.2% for specialpurposes.

Within the broad field of stainless steels there are the hardenable steels containing chromium in the amount of about 12% to 18%. These commonly are hardened by quenching from temperatures on the order of 1800 F. But'unfortunately' this high quenching temperature is inclined todevelop a heat-scale orv surfacetdiscoloration on the stainless steel products. Anda special operation is required either to preclude the formation of the surface scale or remove it, once it appears. Moreover, the final hardening by treatment at a temperature of 1800 F. may cause distortion or other dimensional change. The several difiiculties noted must be reckoned with in any commercial hardening practice. They are of particular consequence in flat rolled products such as plate, sheet, strip or special shapes.

Within the past several years, however, there have been developed stainless steels which are hardenable by heating to a temperature which is sufficiently low to avoid distortion and heat-scale. In this regard reference is made to the austenitic chromium-nickel stainless steels containing a precipitation-hardening agent such as columbium and/or titanium. Mention also is made of the austenitic chromium-nickel stainless steels containing copper in substantial amount. In these several austenitic chromium-nickel stainless steels the nickel of course represents an item of considerable expense. Additionally, it is frequently difficult to procure.

One of the objects of my invention, therefore, is the provision of a precipitation-hardenable stainless steel which is virtually free of nickel, which readily may be hot-worked and cold-worked into a variety of articles of ultimate use, and which subsequently may be precipitation-hardened-by heat-treating methods in which precise control of temperatures and time becomes unnecessary, all to produce products of strength at room temperature and at elevated temperature, with freedom from distortion and heat discoloration.

Referring now more particularly to the practice of my invention, I provide a stainless steel which essentially consists of chromium, copper, nitrogen and carbon in critical amounts, with remainder substantially all iron. Now the chromium amounts to 12.0% to 20.0%, the copper 2.00% to 4.50%, the nitrogen 0.01% to 0.20%, and the carbon 0.01% to 0.20%. In preferred amount the chromium ranges from about 13.50% to 17.50%, the copper about 3.00%, carbon about 0.10%, and the nitrogen 0.02%, with remainder iron.

As permissible impurities in the steel of my invention, manganese is usually presentin the amount of about 0.50%, although this may amount to as much as about 1.00%. Silicon also usually is present in the amount of about 0.50%, although it may amount to as much as 1.00%. Nickel also may be present as an impurity, this generally amounting to as much as 0.20%, although it may reach as much as 0.75%. Phosphorus and sulphur ordinarily are low, although the phosphorus content may be as much as 0.050%, While the sulphur content may be as'much as 0.50% for improving machinability.

In my steel molybdenum may be added for special purposes up to the amount of 5.00%. So, too, there may be special additions of columbium, tantalum and tungsten each in amounts up to about 4.00%. Likewise, vanadiummay be added up to the amount of about 2%. And cobalt may be added up to the amount of about 12%. I: also find that titanium may be added up to about 1% and boron up to 0.050%.

Now in the steel of my invention there is a further limitation on composition: the chromium content is so correlated'with the sumof the carbon and nitrogen contents as to fall within the area ABCD of Fig. 1 of the accompanying diagram. And where molybdenum is employed, I find that the molybdenum content must be considered as an addition to the actual chromium content in making use of the diagram, the chromium and molybdenum thus being considered as chromium equivalents.

The essential composition of the steel of my invention is inevery sense critical. Within the range of composition there is hada steel of the desired room temperature and high temperature properties. With carbon and nitrogen contents in excess of the figures given in terms of percent and in terms of the diagram, however, I find an excessive amount of austenite retained, after the precipitation-hardening treatment. gen contents less than those employed in the steel of my invention, I find insutficient hardening is had. So, too, insufficient hardening is had with a steel of chromium content less than that employed in the steel of my invention. And with a chromium content in excess of that employed in my steel I find that no actual precipitationhardening is obtained.

The steel of my invention is melted in known manner, and in either wrought or cast form is in condition for working, forming and fabrication. For example, the steel in the form of castings is annealed and then machined to desired specification. And the steel in the form of various hot-rolled and cold-rolled products such as plate, sheet, strip, bars, rods, wire and intricate structural members, is annealed and then formed as by pressing, bending, stamping, punching, drawing, and the like, into desired preliminarily formed articles of use. In the fabri- With carbon and nitro- 5 .4 less than about 5 hours and the hardness achieved amounts to Rockwell C3 9-55.

While precipitation-hardening ordinarily is had following the forming and fabricating of the stainless steel articles and products of ultimate use where the hardness is on the order of Rockwell C-38, as noted, it will be understood that where desired the precipitation-hardening treatment may be had prior to fabrication, particularly where very little forming and working is required. Precipitation-hardened stainless steel castings and plate, sheet, strip, bars, rod, wire, special shapes and other converted products accordingly fall within the scope of my invention.

As particularly illustrative of the practice of my invention, I have prepared a series of chromium-copper stainless steels without and with special additions of molybdenum, titanium, tungsten and the like, the chemical analyses of which are given in Table I below, along with the Rockwell hardness in the solution-treated con- 9 dition (1900 F. for /2 hour and oil-quenched) and in TABLE I Composition and hardnes of chromium-copper stainless steels and modifications Rockwell C" Hardness Heat No. C Cr Cu Mo W Ob N V Tl Ni Annealed Harden.

eating operation the various converted products may be brazed, soldered and welded, the latter by are, gas, spot or other known welding methods.

The annealing operation with cast products or with the converted products, is had at a temperature of about 1600 to 2100 F. for periods of up to an hour. Actually, in the steel of my invention this is viewed as a solution-treating step following which the steel is cooled The room temperature mechanical properties of the chromium-copper stainless steel of my invention (Heats 1, 2 and 3 above) as compared with those of a known chromium-nickel-copper stainless steel analyzing 16.45% chromium, 4.22% nickel, 3.55% copper, .042% nitrogen, with .S7% manganese, .018% each of phosphorus and sulphur, .42% silicon, .25% columbinm, and remainder iron, are given in Table II below:

TABLE H Room temperature mechanical properties in air, water or oil, and then formed or fabricated, as noted. The hardness of the metal in the solution-treated condition is on the order of Rockwell C20-38, preferably Rockwell C2834.

In order to enjoy best mechanical properties, my steel,

following the annealing or solution-treating operation, is precipitation-hardened. Thi I achieve by reheating the steel to a temperature of about 800 to 1200 F. for periods of time up to 24 hours. Ordinarily, however,

the duration of the precipitation-hardening treatment is hardened chromium-nickel-copper stainless steel.

It is to be particularly noted that in the precipitationhardened steel of my invention there is achieved a high ultimate tensile strength and high yield strength, this comparing favorably with the high strengths had in the known chromium-nickel-copper precipitation-hardened stainless steels. When hardening by treatment at a temperature of 900 F. for 1 hour it will be seen that the strength of the steel of my invention appreciably exceeds the strength values had with the known precipitation- And thatwith precipitation-hardening had by treatment. at a temperature of 1100 F. for 1 hour, the values had with my steel greatly exceed those achievedwith the known steel. In fact, with the treatment at 1100 F. maximum ultimate tensile strength and maximum yield strength are had in the steels ofmy invention.

The superiority of the mechanical properties of my steel is forcefully emphasized by the graphic comparison given in Fig. 2 of the accompanyingdrawings. In that figure there is shown the hardness had in the chromiumcopper stainless steels of my invention (heat No. 1) as compared with the known chIomium-nickel-copper stainless steel of the prior art (heat designated CrNi-- Cu) as a function'of the hardening temperature with treatment at 1' hour. There also is shown the hardness had in my steel with prolonged precipitation-hardening treatment, i. e. for 58 hours, which may be compared with that had with my steel by a treatment for 1 hour.

It is particularly significant that in the steel of my invention the hardness achieved difiers little, whether the hardening temperature be at 900, 1000", 1100, or even- 1200 F. In the chromium-nickel-copper stainless steel of the prior .art, however, the hardness sharply suffers with over-ageing.

Now as'to the elevated temperature properties of my precipitation-hardened stainless steel, there is given in Table III below a comparison of stress rupture properties of heatsNos. 1 and 3 with those of the known chromium-nickel-copper stainless steel, the stress rupture tests being taken at 800, 900 F. for the comparison and a further test at 1000 F. for one example of my steel.

TABLE III treated condition maybe variously formed, machinedand fabricated, asnby welding. And they may be precipitation-hardened to achieve great ultimate strength and i great yield strength at room temperatures and excellent stress rupture characteristics at elevated temperatures.

Moreover, the precipitation-hardening treatment is not highly critical in terms of time and temperature, but rather the steel of my invention is not subject to overageing, as are certain steels of the prior art.

Inasmuch as many embodiments may be made of my invention and as many changes may be in the embodiments herein set forth, it is to be understood that all. matter described herein or shown in the accompanying drawing is to be interpreted as illustrative and not as a limitation.

I claim as my invention:

1. A chromium-copper stainless steel having precipitation-hardenable properties, said steel essentially consisting of 12% to 20% chromium, 2% to 4.5% copper,

.01% to..20% carbon, .01% to .20% nitrogen, with molybdenum up to 5%, metal of the group consisting of colu-mbium, tantalum and tungsten up to 4%, vanadium up to 2%, cobalt up to 12%, titanium up to 1%, boron upto 0.050%, manganese up to 1.00%, silicon.

up to 1.00%, nickel up to .75%, phosphorus upto.

Stress rupture properties of the precipitation-hardened chromium-copper stainless steel as compared with the precipitation-hardened chromium-nickel-copper stain- +1,100 F.air cool.

1,900F. hr. 0.

900 105, 000 l tin-air cool 900 48,000 .do 1, 000 70, 000 1,900 F. hr. o. q.

Test in progress 1,700 hrs-no fracture. D0. Fractures at 1,000 hrs. Test; in progress 1,100 hrs-no fralgture.

Fractured at 1,000 hrs.

Test in progress 1,600 hours no fracture.

The superior stress rupture properties of my chromiumcopper stainless steel at 800 F. clearly are shown in Table IH. While the precipitation-hardened stainless steel of my invention sustained a load of 155,000 p. s. i. at 800 F. fora period of 1700 hours and more with no fracture, the steel of the prior art fractured at 1000 hours under a load of 89,000 p. s. i. And at 900 F. my steel sustained a load of 105,000 p. s. i. for 1100 hours and more, while the known steel fractured at 1000 hours with .a load of 48,000 p. s. i.' And even at the higher temperature of 1000 F. the steel of my invention, solution-treated and then precipitation-hardened at the higher temperature of 1100 F. for 1 hour, sustained a load of 70,000 p. s. i. for 1600 hours or more, without fracture.

It thus will be seen that I provide in my invention a chromium-copper stainless steel which is particularly suited to precipitation-hardening at comparatively low temperatures in which there are achieved the various objects set forth above, as well as many practical advantages. For example, it will be seen that my steel and the method of hardening the same assures the production of a wide variety of cast and wrought articles and products. The products in the annealed or solutiontion-hardenable properties, said steel essentially consisting of 12% to 20% chromium, 2% to 4.5% copper, .0l% to .20% carbon, .0l% to .20% nitrogen, and remainder substantially all iron, and with the chromium content correlated with the sum of the carbon and nitrogen contents in accordance with the area ABCD of Fig. 1 of the drawings.

3. A chromium-copper stainless steel having precipitation-hardenable properties, said steel essentially consisting of 13.5% to 17.5% chromium, about 3% copper, 01% to .20% carbon, 01% to .20% nitrogen, and remainder substantially all iron, and with the chromium content correlated with the sum of the carbon and nitrogen contents in accordance with the area ABCD of 'Fig. 1 of the drawings.

4. In the production of a precipitation-hardened chromium-copper stainless steel, the art which consists in providing a steel essentially consisting of about 12% to 20% chromium, 2% to 4.5% copper, .0l% to .20% carbon, .01% to .20% nitrogen, with molybdenum up to 5%, manganese up to 1.00%, silicon up to 1.00%, nickel up to .75%, phosphorus up to 0.050%, sulphur up to 0.50%, and remainder substantially all iron, and with the sum of the chromium and molybdenum contents correlated with 7 the sum of the carbon and nitrogen contents in accordance with the area ABCD of Fig. 1 of the drawings; heating the steel to a temperature of 1600 to 2100 F. for periods up to 1 hour and cooling; and reheating to a temperature of 800 to 1200 F, for periods up to 24 hours and cooling.

5. In the production of precipitation-hardened chromium-copper stainless steel articles of manufacture, the art which comprises providing stainless steel essentially consisting of about 12% to 20% chromium, 2% to 4.5% copper, .01% to .20% carbon, 01% to .20% nitrogen, and remainder substantially all iron, and with the chromium content correlated with the sum of the carbon and nitrogen contents in accordance with the area ABCD of Fig. 1 of the drawings; heating the steel to a temperature of about 1600 to 2100 F. and cooling; forming the steel to desired configuration; and reheating to a temperature of about 800 to 1200 F. to effect precipitation-harden- 6. A precipitation-hardened chromium-copper stainless steel essentially consisting of 12% to20% chromium, 2% to 4.5% copper, .01% to .20 carbon, .01% to .20% nitrogen, with molybdenum up to5 manganese up to 1.00% silicon up to 1.00%, nickel up to .75%, phosphorus up to 0.050%, sulphur up to 0.50%, with remainder substantially all iron, and with the sum of the chromium and molybdenum contents correlated with the sum of the carbon and nitrogen contents in accordance with the area ABCD of Fig. 1 of the drawings.

7. A precipitation-hardened chromium-copper stainless steel essentially consisting of about 17.5% chromium, about 3% copper, about .10% carbon, about .02% nitrogen, and remainder substantially all iron.

8. A precipitation-hardened chromium-copper stainless steel essentially consisting of about 15.5% chromium, about 3% copper, about .10% carbon, about .02% nitrogen, about 2% molybdenum, and remainder substantially all iron.

9. A precipitation-hardened chromium-copper stainless steel essentially consisting. of about 13.5% chromium, about 3% copper, about .10% carbon, about .02% nitrogen, about 4% molybdenum, and remainder substantially all iron.

10. A precipitation-hardened chromium'copper stainless steel essentially consisting of about 14.5% chromium, about 3% copper, about 1.5% molybdenum, about 1.5% tungsten, about .10%ca1'bon, about .02% nitrogen, and remainder substantially all iron.

11. A precipitation-hardened chromium-copper stainless steel essentially consisting of about 15% chromium, about 3% copper, about 2.5% tungsten, about .10% carbon, about .02% nitrogen, and remainder substantially all lI'OIl.

References Cited in the file of this patent FOREIGN PATENTS 475,895 Great Britain Nov. 29, 1937 

1. A CHROMIUM-COPPER STAINLESS STEEL HAVING PRECIPITATION-HARDENABLE PROPERTIES, SAID STEEL ESSENTIALLY CONSISTING OF 12% TO 20% CHROMIUM, 2% TO 4.5% COPPER, .01% TO .20% CARBON, .01% TO .20% NITROGEN, WITH MOLYBDENUM UP TO 5%, METAL OF THE ROUP CONSISTING OF COLUMBIUM, TANTALUM AND TUNGSTEN UP TO 4%, VANADIUM UP TO 2%, COBALT UP TO 12%, TITANIUM UP TO 1%, BORON UP TO 0.050%, MANGANESE UP TO 1.00%, SILICON UP TO 1.00%, NICKEL UP TO .75%, PHOSPHORUS UP TO .050%, SULPHUR UP TO 0.50%, AND REMAINDER SUBSTANTIALLY ALL IRON, AND WITH THE SUM OF THE CHROMIUM AND MOLYBDENUM CONTENTS CORRELATED WITH THE SUM OF THE CARBON AND NITROGEN CONTENS IN ACCORDANCE WITH THE AREA ABCD OF FIG. 1 OF THE DRAWINGS. 